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Edit #2

Confirmed as a bug by WRI tech support.

And to get an item of 5Ms wide instead of 4Ms I had to set the item size to 5.7!

Note that using lower case "m" made no difference. So either the documentation is wrong, or it is incomplete, or the implementation is broken.

Note that using lower case "m" made no difference. Note that further investigation shows that the word wrapping has an effect here. With just 5Ms the item size almost works.

Initial summary: So either the documentation is wrong, or it is incomplete, or the implementation is broken.

Subsequent summary: the problem seems to reside with how Mathematica determines the font being used when doing the size calculations. It clearly responds when StandardForm is changed but does not respond to local font usage (apparently). Otherwise an item size of x/CurrentValue["FontMWidth"] should surely deliver an item xMs wide.

And to get an item of 5Ms wide in all cases I had to set the item size to 5.7!

And to get an item of 5Ms wide instead of 4Ms I had to set the item size to 5.7!